In LED dimming systems, conventionally the LED is turned on and off between ground and its forward voltage to fulfill dimming function. The abrupt change of voltage may arise of the danger of overstressing the LED and other peripheral components. For a system whose power is LED's output, it will temporarily shutdown during the LED's off period. This causes limits when designing such circuits. In further detail, as shown in FIG. 1, a conventional LED dimming circuit includes a boost integrated circuit (IC) 10 to boost a battery voltage Vbat into a driving voltage Vo for a LED and a functional IC 12 connected to the anode of the LED for dimming control. Dimming is realized through a switch M serially connected to the LED, for which the functional IC 12 provides a dimming signal Dpwm to switch the switch M in order to adjust the average current Iled of the LED, thereby achieving dimming control for such as bright, dim and flashing. Circuits and operations for the boost IC 10 and the functional IC 12 have been mature and need not to be discussed in detail herein. When the functional IC 12 turns off the switch M to cut off the current Iled, since no path to ground exists, the output VOUT of the boost IC 10 will endure a very high voltage due to the continuously charged capacitor Cout connected at the output VOUT, and thereby push the boost IC 10 into its over voltage protection mode. When the functional IC 12 turns on the switch M again, the charge stored in the capacitor Cout will rush into the LED, and the LED will endure a large voltage before the output voltage Vo drops to the LED's normal forward voltage again. In this way, although the functional IC 12 can work when the LED is off, the boost IC 10, the functional IC 12 and the LED are overstressed by a very high voltage and this causes quality concerns. For those functional ICs sensitive to power, this method may even cause errors during dimming period.
FIG. 2 shows another possible solution for a battery powered LED flashlight dimming system, in which the functional IC 12 is powered separately, e.g. by another battery Vbat2. When the LED is on, the functional IC 12 enables the boost IC 10 to boost the battery voltage Vbat1 into a driving voltage Vo equal to the normal forward voltage of the LED. When the LED is off, the functional IC 12 disables the boost IC 10, and thus the driving voltage Vo will not increase to the extent that the boost IC 12 enters its over voltage protection mode. By doing this, not entering the over voltage protection mode makes the whole system safer and prolongs the utility time of the LED. However, this approach also has two drawbacks. (1) For low battery power, e.g. 0.9V, most functional ICs are unable to work under such low supply voltage. This limits the application of the solution. (2) The LED is dimmed between the normal forward voltage Vf and a ‘low’ voltage (i.e. Vbat1−VD). The voltage drop during dimming is not minimized and the LED is still overstressed by some unnecessary abrupt voltage change. For example, assuming that Vbat1=1.5V, VD=0.7V and Vf=3.6V, the LED will be overstressed by an abrupt voltage change ΔV=Vf−(Vbat1−VD)=3.6V−(1.5V−0.7V)−2.8V when it is switched from on to off, or from off to on. This abrupt voltage change ΔV increases with the decrease of the battery voltage Vbat1. The abrupt voltage change will shorten the LED's life time.
Therefore, it is desired a dimming circuit and method for LEDs that prolongs the LED's life time while maintains a certain low voltage when the LED is off to support other functional circuits.